This invention relates to a variable ratio transmission.
Various forms of variable ratio transmission exist. One simple form of such a transmission comprises a pair of discs which are mounted on parallel but displaced shafts and which have curved engaging faces. The shafts can be moved closer together or further apart, varying the point of contact between the discs. Theoretically, there is an infinitely thin pitch line on each disc which determines the ratio of transmission. In reality, the pitch line has a finite thickness, and contact between the discs takes place over a measurable area, rather than at a point. Accordingly, there cannot be true rolling contact between the discs, and some degree of slippage must occur, leading to heat buildup and wear. Apart from this, the sliding coefficient of friction between the discs is usually lower than the static co-efficient of friction, so that the torque which can be transmitted between the discs is reduced.
Transmissions such as that described above are generally restricted to use in low torque applications. Variable ratio transmissions for motor vehicles, for example, tend to be more complicated and may be relatively inefficient. For example, hydraulic torque converters used in motor vehicles dissipate a considerable amount of energy. Other forms of continuously variable transmission which have been proposed employ belts or chains which are driven by variable diameter pulleys. Such transmissions suffer from various practical problems which have limited their applicability to relatively low powered vehicles. The longevity of such systems is also suspect.